Contact lenses for continuous wear over a long term are made of silicone rubber prepared from polydimethyl siloxanes. Since the silicone rubber contact lenses are very water-repellent and greatly different from the cornea in thermal properties, such as thermal conductivity and thermal diffusivity, they give a foreign body sensation, particularly a burning sensation despite having oxygen permeability. Contact lenses made from silicone rubber are uncomfortable to wear. Further, the silicone rubber is soft and elastic, making it difficult to conduct precise mechanical treatments such as cutting, grinding, and polishing. Many attempts for making the surface of silicone rubber lenses hydrophilic have been undertaken, but no completely satisfactory contact lens has been developed. High water content contact lenses are usually made of poly-N-vinylpyrrolidone polymers. Since the high water content contact lenses contain about 60% to about 80% by weight of water, they have the disadvantages that they are (a) weaker in quality of material than low water content contact lenses, (b) easily contaminated with inorganic and organic compounds in tears which penetrate and accumulate into the lenses during the use, and (c) bad in maintenance of lens contour due to the evaporation of water during the use and, therefore, the refractive power thereof easily changes.
Reactive silicone-hydrogel formulations are used to make extended wear soft contact lenses due to their relatively high oxygen permeability, flexibility, comfort, and reduced corneal complications. Conventional hydrogel materials (e.g. 2-hydroxyethyl methacrylate, HEMA) by themselves have poor oxygen permeability and they transport oxygen to the eye through the absorbed water molecules. Water has low oxygen permeability, also called the Dk value, which may be expressed in Barrer, wherein 1 Barrer=10−11 (cm3 O2) cm cm−2 s−1 mmHg−1 where “cm3 O2” is at a quantity of oxygen at standard temperature and pressure and where “cm” represents the thickness of the material and “cm−2” is the reciprocal of the surface area of that material. The Dk of water is 80 Barrer. Upon exposure to atmospheric air for long periods, these lenses are slowly dehydrated and the amount of oxygen transported to the cornea is reduced. Eye irritation, redness and other corneal complications can result and hence restrict use of the lenses to limited periods of wear. But blending reactive silicone monomers with conventional monomers as a potential solution have been marred by compatibility issues.
A possible solution to this problem is to make the silicone monomer inherently hydrophilic by incorporating hydrophilic units on the monomer. One approach to provide hydrophilic silicone monomers is to polymerize the organo-modified silicone monomer with organic monomers in the presence of a cross-linker. Examples of prior attempts of providing hydrophilicity include those described in U.S. Pat. Nos. 4,260,725; 5,352,714; 5,998,498; 6,867,245; 6,013,711; 6,207,782; 7,601,766; 7,557,231; 7,732,546; 7,781,558; 7,825,273, which are each incorporated herein by reference. This approach leads to a large number of unreacted monomers due to unregulated viscosity build-up that requires extracting the leachable monomers from the matrix by water-isopropanol solvent mixtures, which leads to increased processing costs. Further, the silicone hydrogel formulations made by these methods still fail to exhibit significant wettability.
Methacryloxypropyltris-(trimethylsiloxy)silane monomers have been used to prepare silicone-containing hydrogels. The (meth)acryloxypropyltris-(trimethylsiloxy)silane is hydrophobic and is used in preparing polyurethane-silicone polymers. These polyurethane-silicone polymers contain blocks of hydrophobic silicone. Contact lenses made from these polymers may cause eye discomfort because of the hydrophobic regions within the polymer.
Silicone-hydrogels are typically made from acrylate or methacrylate functionalized silicone monomer that are polymerized with hydrophilic organic monomers, such as hydroxyethyl methacrylate (HEMA), N-vinylpyrrolidone (NVP) and other monomers such as methyl methacrylic acid (MA), and N,N-dimethylacrylamide (DMA), in the presence of crosslinkers and free radical or photoinitiators. Crosslinking agents generally have two or more reactive functional groups at different sites of the molecule. Typically, these sites contain polymerizable ethylenic unsaturation groups. During polymerization to form the silicone-hydrogel, they form a covalent bond with two different polymer chains and form a stable three-dimensional network to improve the strength of the polymer. Crosslinking agents conventionally used in contact lenses include ethylene glycol dimethacrylate and trimethyloylpropane trimethacrylate. Other useful crosslinking agents include diethyleneglycol dimethacrylate, bisphenol A dimethacrylate, diglycidyl bisphenol A dimethacrylate, and dimethacrylate-terminated polyethylene glycol, and reactive linear polyether modified silicones. The oxygen permeability of these silicone-hydrogels is affected by the chemical structure of the acrylate or methacrylate functionalized silicone monomer and choice of the other monomers containing reactive carbon-carbon double bonds that are used in preparing the crosslinked polymer.
Silicone-hydrogel contact lens materials are typically made using either hydrophobic mono-functional silicone monomers or multi-functional hydrophilic silicone monomers followed by secondary surface treatment. Mono-functional silicone monomers are often used in the contact lens industry over multi-functional silicone monomers since the latter lead to increased rigidity in the lenses made therefrom. The known mono-functional silicone monomers, however, may have deficiencies. For example, monofunctional siloxane-polyether (meth)acrylates are susceptible to air oxidation. Monofunctional (meth)acryloxy functional siloxanes that contain 1,4-substition on the (meth)acryloxy group to the siloxane group on a six-member ring, such as for example, (meth)acrylic acid 2-hydroxy-4-[2-bis-(trimethylsiloxy)methylsilanyl-ethyl]-cyclohexyl ester, form highly ordered copolymers which may inhibit the permeability of oxygen through the silicone-hydrogel. 1,3-substitution of the (meth)acryloxy group to the siloxane group on a six-member ring, such as for example, (meth)acrylic acid 2-hydroxy-5-[2-bis-(trimethylsiloxy)methylsilanyl-ethyl]-cyclohexyl ester, form less order copolymers, but the moderate polarity of the (meth)acryloxy group may affect the hydrophilic properties of the silicone-hydrogel.
The state of this art for soft contact lenses, including the silicone-based materials described in the above mentioned patents, still possess major shortfalls like formulation compatibility, sub-optimal surface wettability, lipid deposition, the need for compatablizers in preparing the polymer, internal wetting agents, or post processing treatments such as “plasma oxidation” surface treatments. These approaches can decrease oxygen permeability or require the use of compatabilizers, which add costs during the manufacturing process. There remains a need for hydrophilic silicone monomers with advantageous wettability and oxygen permeability that can be used to make contact lenses without the drawbacks and expensive surface treatments necessary with the native silicone containing materials of the current state of art.
Hence, there remains a need for hydrophilic silicone monomers with inherently advantageous wettability, stability to air oxidation, high oxygen permeability and high solubility in the other reactive monomers used to make the polymer without the need for compatibilizers.